These
microsatellites have a high mutation, or error, rate as they are passed
from generation to generation, making them a useful tool to study when
two populations diverged.

Researchers
from Stanford University, US, and the Russian Academy of Sciences
compared 377 microsatellite markers in DNA collected from 52 regions
around the world.

Analysis
revealed a close genetic kinship between two hunter-gatherer
populations in sub-Saharan Africa - the Mbuti pygmies of the Congo
Basin and the Khosian bushmen of Botswana.

First migration

The researchers believe that they are "the oldest branch of modern humans studied here".

The
data also reveals that the separation between the hunter-gatherer
populations and farmers in Africa occurred between 70,000 and 140,000
years ago. Modern man's migration out of Africa would have occurred
after this.

An
earlier genetic study - involving the Y chromosomes of more than 1,000
men from 21 populations - concluded that the first human migration from
Africa may have occurred about 66,000 years ago.

The
small genetic diversity of modern humans indicates that at some stage
during the last 100,000 years, the human population dwindled to a very
low level.

It was out of this small population, with its consequent limited genetic diversity, that today's humans descended.

Small pool

Estimates of how small the human population became vary but 2,000 is the figure suggested in the latest research.

"This estimate does not preclude the presence of other populations of Homo sapiens sapiens (modern man) in Africa, although it suggests that they were probably isolated from each other genetically," they say.

The authors of the study believe that contemporary worldwide populations descended from one or very few of these populations.